Protein metabolism is highly regulated in physiological and pathophysiological states such as diabetes, starvation, etc. Both within the whole organism and at the cellular level, proteins exist in a dynamic state, maintained by the relative balance of protein synthesis and degradation. The bulk of cellular protein degradation is via the ubiquitin proteolytic pathway which provides both selectivity and specificity. Herein, cellular proteins via their internal lysine residues are tagged with ubiquitin and degraded via the proteasome. Recent evidence demonstrates (1) that this pathway exists within the cell nucleus and (2) a new pathway whereby ubiquitination is targeted to the N-terminus of a target protein. Our overall hypothesis is that cellular protein degradation is regulated in part by its loci of activity within the cell and by distinct recognition mechanisms. The aims of the present proposal are thus (1) to analyze the motif, identify the ubiquitin ligase(s), and determine the physiological significance of N-terminus dependent ubiquitination of cellular proteins;(2) to define the subcellular site and mechanism(s) responsible for degradation of several interacting muscle development/differentiation factors;and (3) to determine the site and mechanism(s) of glucocorticoids in the accelerated protein catabolism in muscle. These studies will be carried out using a variety of physiological, pharmacological, biochemical, molecular and cellular systems. Ultimately, the ability to modulate protein metabolism in physiological and pathophysiological states requires a detailed mechanistic understanding of the pathways of protein metabolism, the subject of the present proposal. PUBLIC HEALTH RELEVANCE: Protein metabolism is highly regulated in both health and disease states, such as diabetes, starvation, etc. The ability to modulate this complex process requires a detailed mechanistic understanding as described in this proposal. Our studies on the regulation of protein metabolism will provide significant insights for development of therapeutic strategies.